There's a few things you could be referring to. I'm guessing it's possibility 3, which I also will link to a cool tool for. I suppose I could just paste that single link as a reply, but I think reading through the others may be instructive or helpful for some people reading this, so why not?

Possibility 1:

In order to calculate the amount of time something would cost you if you had instead saved the money and invested it, MMM's rough calculation is:

to calculate a weekly expense compounded over ten years, multiply the price by 752

for a monthly expense, multiply by 173

(From this article:

http://www.mrmoneymustache.com/2011/04/15/getting-started-3-eliminate-short-termitis-the-bankruptcy-disease/ and referenced here:

www.mrmoneymustache.com/2012/07/15/two-fun-tools-from-the-mmm-software-department/)

Possibility 2:

To find how long you'd have to save for a one time expense, you need to divide the cost of the item by your hourly true rate. This will give you the number of hours extra you'll have to work at the end of your career to have that thing.

Possibility 3:

To find out how long you'll have to work for a reoccurring expense, first figure out how much that thing will cost you annually (if it's a once a year thing, you're done. Weekly, multiply by 52. Daily multiply by 365, etc.). Then multiply that annual cost by 25 (if you're shooting for a 4% SWR) or 33 (if shooting for a 3% SWR), which will give you the total amount you need in your stache to cover that annual expense. Then divide that number by your true hourly wage, which will give you how much longer you have to work to get that number.

This is a great tool to help you quickly do that last possibility:

TLDR: http://networthify.com/calculator/real-costIf none of those was quite it, please clarify and we'll try to figure it out.

Hope that helps!